1. Technical Field
The invention relates to a method for manufacturing a stent delivery system. More particularly, the invention relates to a method for manufacturing a stent delivery system having a holder interlocking and interfering with a stent.
2. Related Art
Stents are commonly used to treat stenosis of various arteries. Where blood vessels are clogged or narrowed by substances that restrict blood flow, stents are delivered into such vessels and expanded to dilate blood vessels or maintain the dilated state of blood vessels. Expansion of stents may be made with or without the aid of a balloon. Balloon-expandable stents are expanded by inflating a balloon disposed beneath a stent. On the other hand, self-expandable stents are capable of expanding without the use of a balloon. For this purpose, self-expandable stents are generally made from shape memory or spring metal, such as nitinol or stainless steel, so that self-expandable stents are able to expand from a compressed state upon removal of pressures applied thereon.
Determining the proper stent to use is the first step to deploying a stent. The proper stent is determined, in part, based on where a stent is to be deployed. For example, balloon-expandable stents are suitable for coronary arteries, whereas self-expandable stents are more suitable for peripheral arteries. However, the uses of balloon-expandable stents and self-expandable stents often overlap, and each type of stent may be used in a variety of applications. In addition, long lesions or tandem lesions require long coverage. Multiple short stents or a single long stent may be implanted in long lesions or tandem legions.
Once deployed into a human body such as an artery, stents generally remain as permanent implants. Accordingly, stents need to comply with high quality standards and minor manufacturing defects on the stents may result in the manufacturing rejection of the stents. Stents are generally manufactured through complicated and labor-intensive processes. Stent manufacturing processes include laser cutting spring metal to form multiple, interconnected struts of a stent; sandblasting a stent to eliminate debris generated from the laser cutting, and electropolishing processes. Because of these processes, it is more difficult to manufacture long stents with high precision and quality than short stents, in part because a long stent is prone to manufacturing defects along the length compared to a short stent. Where a long stent is rejected due to manufacturing defects, material costs and manufacturing expenses substantially increase.
Although defect-free long stents may be successfully manufactured, conventional stent delivery systems tend to improperly deploy long stents. This is particularly a problem in conventional stent delivery systems where uneven, high forces are applied at the proximal end to push a long stent out of the delivery system upon deployment. FIGS. 1A and 1B illustrate a conventional stent delivery system 1 for a self-expandable stent 10. The stent delivery system 1 includes the self-expandable stent 10, a holder 18 and a sheath 8. The sheath 8 radially constrains the stent 10 during delivery and is retracted when the stent 10 needs to be deployed as shown in FIG. 1B. The holder 18 is disposed beneath the stent 10 and supports the stent 10 during delivery. The stent 10 expands from a compressed state to an expanded state as shown in FIGS. 1A and 1B. The stent delivery system 1 is mounted on one end of a delivery catheter 50. The delivery catheter 50 has an outer tube 8 functioning as a sheath and a core 4, which longitudinally extends from a proximal end 20 to a distal end 22. The core 4 is connected to a hub 2 at the proximal end 20 and to a holder 18 at the distal end 22. The outer tube 8 is connected to a handle 7. A physician deploys the stent 10 by pulling the handle 7 towards the hub 2. As the outer tube 8 is retracted by pulling the handle 7, the stent 10 is exposed and starts expanding. The stent 10 is fully deployed when the handle 7 reaches the hub 2. However, the stent delivery system 1 often experiences problems when the stent to be deployed is long. Deployment of long stents often requires high concentrated force particularly at the proximal end 22 to push the long stent out of the stent delivery system 1. This frequently results in improper or inaccurate deployment of the long stent.
In addition to improper deployment of long stents, the stent delivery system 1 presents other disadvantages as well. One disadvantage is that it is difficult to reduce the size of the stent delivery system 1. It is generally desirable for most stent delivery systems to have a low profile. Stent delivery systems that have lower profiles reduce possible damage to blood vessels during delivery and deployment of the stent. Further, stent delivery systems with lower profiles may be able to get to small and/or tortuous blood vessels. However, the sheath 8 substantially increases the overall profile of the stent delivery system 1. When the sheath 8 is retracted, the stent 10 may unexpectedly and/or uncontrollably move. As previously stated, because the stent 10 is made from spring metal, it tends to expand upon retraction of the sheath 8. This makes it difficult for physicians to accurately position the stent 10. Various attempts have been made to address this problem. For example, structures such as rings and shafts may be added to an inner holder adjacent the proximal end. These structures may engage the proximal end of a stent in order to longitudinally restrain the stent. When the sheath is retracted, the distal end of the stent is first exposed into the blood vessel. Because the proximal end of the stent is temporarily restrained by these structures, the stent may not abruptly move in response to the retraction of the sheath. However, the structures, such as rings and shafts, may be counterproductive to accurate deployment of stents because they trap the stent which must be expanded. In addition, such structures require sophisticated design, which increases manufacturing expenses.
The stent delivery system 1 may not be optimal for delivering and deploying drug coated stents. The stent 10 may include drug coatings on the outer surface thereof. Drugs may be coated on the stent 10 for various purposes. For example, drugs may prevent the formation of scar tissue on the vessel walls or reduce restenosis. Contrary to these benefits, some drug coatings may cause unfavorable consequences if applied improperly. For example, drugs, such as scar prevention drugs, may be highly incompatible with blood. Thus, when drugs that are coated on the stent 10 come into contact with blood, the drugs may cause problems such as blood clots. For this reason, it is desirable that drugs are disposed only on the outer surface of the stent 10. Because the outer surface of the stent 10 is pressed against the vessel walls upon expansion, blood does not flow between the outer surface of the stent 10 and the vessel walls. However, the conventional stents 10 usually contain drug coatings on the sides and inner surfaces which come into contact with the blood. Drug coating material is typically sprayed on a stent 10 when it is in an expanded state. Because the stent 10 is self-expandable, it is generally not possible to spray the drug coating material on the compressed stent 10 since the outer surface of the stent 10 is constantly pressed against the inner surface of a transfer tube or a sheath 8 when it is compressed. When the drug coating material is sprayed on the expanded stent 10, it easily covers the sides and inside surfaces of the stent 10 through the openings between the struts of the stent 10. Further, the stent has relatively large openings when it is expanded. This reduces the efficiency of spraying because a substantial amount of sprayed drugs passes through the openings.
Even if drugs may be adequately sprayed on the expanded stent 10, they may be lost in the course of manufacturing (e.g., loading into the delivery system) and the deployment processes of the stent 10. The stent 10 must be compressed, for example, by rolling it down to a smaller diameter. During this compression process, shear force or mechanical trauma is applied to the stent 10 and a substantial amount of the drug coating may be lost. Further, when the stent 10 is pushed into the sheath 8 and the sheath 8 is later retracted rearward to deploy the stent 10, a substantial portion of the drug coating may be lost. Accordingly, there is a need for a stent delivery system that overcomes the foregoing drawbacks.